1. Technical Field
The present invention relates to a signal conversion device, a video projection device, and a video projection system.
2. Related Art
Recently, as more and more video signals are digitized, various standards are prescribed such as standards related to HDV (High-Definition Video), HDMI (High-Definition Multimedia Interface) and recording of video digital signals. These standards enable expression of broader color reproduction ranges, broader dynamic ranges and many more gradation levels, in anticipation of possible enhancement of performance of the current video output devices and image pickup devices and future improvement in their capability.
As video output devices conformable to such standards, there are devices that express gradation with analog signals or express gradation by using PWM (Pulse Width Modulation) control, such as plasma displays and liquid crystal display devices. In the case of expressing gradation with analog signals, though it is theoretically possible to realize unlimited gradation levels, practically, it is difficult to accurately reproduce multiple gradation levels. Meanwhile, in the case of expressing gradation by PWM control, the moving speed of a physical movable mechanism such as a mirror reaches its limit, making it difficult to acquire high resolution. It is also difficult to increase the pixel density to perform on-off control of pixels at a high speed.
On the other hand, unlike a direct-view display, a projector as a video projection device can easily superimpose plural projection videos on the screen. Thus, using this characteristic, the same video is superimposed and displayed on the screen from plural projectors, thereby improving luminance. For example, JP-A-2003-125317 discloses a technique of superimposing projection videos from two projectors and thereby increasing the number of gradation levels.
However, in the technique disclosed in JP-A-2003-125317, videos are projected in such a manner that the two projectors project substantially the same quantity of light for each gradation level or in such a manner that the quantity of light projected by one projector for each gradation level is substantially an integral multiple of the quantity of light projected by the other projector for each gradation level. Therefore, in consideration of non-linearity of light modulation elements of the projectors and non-linearity of the characteristics of the eyes of a person viewing the video, the multi-gradation effect may be low even when the video is projected as disclosed in JP-A-2003-125317.